Polyurethane chemistry is currently utilized in producing numerous types of adhesive products. Two-component systems, one-component moisture-cured systems, one-component heat-cured systems and one-component thermoplastic hot melt systems have been used to produce adhesive compositions.
Two-component systems require metering and mixing equipment and heated or non-heated fixturing until substantial curing takes place. The final properties of the adhesive compositions produced by such systems depend on the accuracy of the mix ratio and the efficiency of mixing. U.S. Pat. No. 4,336,298 to Schwarz discloses a two-component polyurethane adhesive system useful for bonding together fiberglass reinforced polyester panels used in the construction of automobiles. The adhesive system comprises a base component containing an aromatic diisocyanate, and a hardener component containing a polyester or polyether triol, paraphenylene diamine or orthophenylene diamine, and preferably, a tin catalyst. The base component and hardener component are mixed together just prior to bonding. The panels to which the adhesive is applied are assembled and held together by clamping until the adhesive sets.
One-component, moisture curable adhesive compositions cure fairly slowly due to dependence on relative humidity and the rate of diffusion of atmospheric moisture into the bondline. Further, fixturing may be required. U.S. Pat. No. 4,511,626 to Schumacher teaches one-component, moisture-curable polyurethane adhesive, sealant and coating compositions useful in bonding articles to substrates. The compositions contain (a) prepolymer(s) derived from 4,4'-diphenylmethane diisocyanate, or isocyanate functional derivative(s) of 4,4'-diphenylmethane diisocyanate, and polyol(s) containing hydroxyl groups, and (b) bis[2-(N,N-dialkylamino)alkyl]ethers. The compositions disclosed by Schumacher require several days for cure-through by atmospheric moisture.
One-component, heat-curable adhesive compositions require sustained heating and possibly fixturing to cure. Heat sensitive substrates cannot be used. U.S. Pat. No. 4,647,646 to Hardy et al. discloses one-component, heat-curable polyurethane adhesive compositions useful for bonding nylon or polyester flock to a primed polychloroprene, SBR or EPDM base layer. The composition is prepared as follows. A urethane prepolymer is prepared by reacting an isocyanate compound and a polyol compound. Two thirds of the isocyanate terminals are then reacted with a ketoxime blocking agent. The remaining one third of the isocyanate terminals are reacted with an amine group of a trifunctional amino compound. Following application, the adhesive is heated and the blocking agent is released and the deblocked isocyanate groups are free to react with available amine groups on neighboring prepolymer molecules resulting in curing and crosslinking. Although Hardy et al. claims lower bake temperatures and shorter cure times than the prior art, sustained heating is required after application of the adhesive composition, and use with heat sensitive substrates may not be possible. However, since flocking adhesives are usually sprayed, brushed or rolled as solutions, very thin layers are employed which require less intense curing conditions.
One-component, thermoplastic hot melt adhesive compositions are non-reactive and therefore have inferior heat resistance after application. U.S. Pat. No. 4,608,418 to Czerwinski et al. discloses processes for forming one-component thermoplastic polyurethane hot melt compositions by blending one or more polyisocyanate compounds with one or more hydroxyl terminated polyols, one or more chain extenders and one or more plasticizers. No chemical curing of the composition occurs after its application. Adhesive properties develop as the material cools and resolidifies.